There exist methods for converting lignocellulosic biomass into fermentable C5 and C6 sugars, including supercritical hydrolysis, acid hydrolysis, and enzymatic hydrolysis.
In acid hydrolysis, strong mineral acids like sulfuric or hydrochloric are used as solvents to convert cellulosic solids into liquid phase sugars. Current acid hydrolysis technologies suffer from technological flaws and economical drawbacks. Technically, acid processes using dilute acids require high temperatures and pressures that can cause manufacturing equipment to suffer from corrosion, and the removal of the acids requires large amounts of neutralizing agents. Acid processes using concentrated acids can operate at lower temperatures and pressures, but they require significant capital and operating expenses for the removal and recycling of the acid. Accordingly, dilute acid systems require significant operating expense for the maintenance and upkeep of the system components, while concentrated acid systems required significant capital expense for sophisticated acid recovery systems and special materials during construction. Acid purchasing itself is a significant cost to the system.
The basis of the technology is well known, although it is not practiced on a large scale today due to the inherent economic challenges. Some are pursuing acid hydrolysis focusing on concentrated acids and special recovery systems. Some of these acid technologies have the potential to somewhat address the operating cost concerns of this process by recovering ever increasing amounts of the used acid, however, these extensive recovery systems require significant capital outlays causing the resulting sugar to be uneconomical.
Enzymatic hydrolysis involves developing biological catalysts that can solubilize hemicellulose or cellulose inherent in lignocellulosic biomasses. While there have been advances in this field (it has been researched for several decades), the challenges are twofold: economics and feedstock flexibility. Enzymatic hydrolysis faces a different set of challenges currently in the marketplace. First, the cost of the enzymes themselves is high. Second, enzymatic operations also face the question of building their own enzyme production facility or paying for and organizing several shipments of enzymes each week—with either scenario requiring substantial investment in upstream capital expense. Third, enzymes can also take days to break down biomass, causing high capital expenses. While the enzymatic routes produce very little by-products, the hydrolysis rates are very slow, necessitating large reaction vessels and large quantities of expensive enzymes. Enzymes are also relatively inefficient at conversion, so, the biomass is usually pretreated by steam explosion, conventional or specialized milling techniques, or using digester technology. Lastly, enzymes may need to be optimized for different types of biomass—a process that can take a very long time, cost significant amounts of money, and have an uncertain outcome.
It would be desirable to develop processes that utilize all types of feedstocks, including switchgrass, corn stover and cobs, wheat straw, and softwoods, especially feedstocks that can be stable to autohydrolysis. It would also be desirable to develop supercritical hydrolysis processes that utilize only water at elevated temperatures and pressures to quickly breakdown cellulose, because they would use no significant consumables and could produce much of their own process energy. The methods and compositions of the present invention are directed toward these, as well as other, important ends.